fine-tuning in package filter

[JorderGomes]

• src/main/java/org/repodriller/filter/range/LastMonths.java:13: SingularField: Perhaps 'sixMonthsAgo' could be replaced by a local variable.

• LooseCoupling: Avoid using implementation types like 'LinkedList'; use the interface instead

  src/main/java/org/repodriller/filter/range/BetweenDates.java:25:
  src/main/java/org/repodriller/filter/range/DailyCommits.java:27:
  src/main/java/org/repodriller/filter/range/DailyCommits.java:39:
  src/main/java/org/repodriller/filter/range/LastMonths.java:22:
  src/main/java/org/repodriller/filter/range/MonthlyCommits.java:28:
  src/main/java/org/repodriller/filter/range/MonthlyCommits.java:40:

• src/main/java/org/repodriller/filter/range/SinceCommit.java:23: LooeCoupling: Avoid using implementation types like 'LinkedList'; use the interface instead

• src/main/java/org/repodriller/filter/range/DailyCommits.java:16:UnusedPrivateField: Avoid unused private fields such as 'log'.

• src/main/java/org/repodriller/filter/range/DailyCommits.java:45:UselessParentheses: Useless parentheses.

• src/main/java/org/repodriller/filter/range/MonthlyCommits.java:46: UselessParentheses: Useless parentheses.